Time period of building

[AniMhj]

Dear everyone,
I am analysing RC building with 26 stories in ETABS. From its modal analysis, the time period of fundamental mode of the building is more than 5 seconds. Is it OK to get such a high time period?

 

[1503-44]

2.5 seconds max. Stiffen it up.

https://www.routledgehandbooks.com/doi/10.1201/9781315374468-4

A rule of thumb for preliminary design is that the building period equals the number of stories divided by 10; therefore, period is primarily a function of building height. The 60-story Citicorp office building in New York has a measured period of 7 s; give it a push, and it wall sway slowly back and forth completing a cycle every 7 s. Other factors, such as the building's structural system, its construction materials, its content, and its geometric proportions, also affect the period, but height is the most important consideration

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[ThomasH]

A 26 storey concrete building with natural frequency of 0.2 Hz (1 / 5 sec = 0.2 Hz). What does the mode look like? Are you sure that the model is correct?

 

[Luceid]

Agree with above - this period seems extremely long. What is the lateral system for the structure?

 

[271828]

You could double-check the computer output by:

1. Apply the weight of the building laterally
2. Run a static analysis
3. Get the deflection, Delta, in inches at the top
4. Natural frequency approximately equals 0.18*sqrt[ (386 in./s2) / Delta ]
5. Natural period = 1 / Natural Frequency

When I've done this, the result has been pretty close, so it's a good sanity check on the computerized analysis.

 

[ThomasH]

A comment on 271828's post.
If you look at formulas for deflection due to selfweight and natural frequency, for a beam they have the same input data. So you can rewrite the formulas so one parameter becomes the function of the other. That is what 271828 shows you.

The natural frequency can be a good indicator of how stiff a building is. Your building seems very "soft" in my opinion. The next thing is the mode shape, is it a global mode? I don't know the geometry but a cantilever or something similair is what I would expected. If it is a local effect, is it a real effect or a problem with the model?

 

[milkshakelake]

You should check the deflected shape and make sure all your supports are in place. Sometimes, a disconnected node (like a beam or column not connected to anything) can really throw off the modal analysis.